Heretofore, the non-contact power supply system which supplies power without electrical contact has been provided with a circuit which detects matter mixed into a magnetic field between the power supply device and the power receiving device as foreign matter in some cases. This is because when foreign matter of a conductor is mixed into a magnetic field, an eddy current arises in the foreign matter, and then the foreign matter sometimes generates heat under the influence of Joule heat generated by the eddy current. When the amount of heat generation of the foreign matter is large, there has been a possibility that devices and cases in the non-contact power supply system have been damaged. Since the strength of the magnetic field output from the power supply device becomes high particularly in boosting charge, the amount of heat generation of the foreign matter also becomes large, and thus the presence of the foreign matter has posed a problem in many cases.
As the circuit which detects foreign matter, a circuit has been proposed which judges the presence or absence of foreign matter based on whether the amplitude of a voltage induced to the power reception side is less than the standard value, for example (Patent Literature 1.). When there is foreign matter, the loss of power occurs due to the eddy current of the foreign matter, which reduces the power transmission efficiency. Therefore, when the amplitude of the voltage on the power reception side decreases to be less than the standard value, it is judged that there is foreign matter.